Method of forming a rivet

ABSTRACT

A hollow rivet is formed in sheet metal by first forming a dimple or hollow boss and then converting the dimple into the hollow rivet. Attenuation of sheet metal to form the dimple is carried out by initially stretching metal and subsequently squeezing the sloping wall of the dimple to extrude the sheet metal in opposite directions. The squeezing step to complete the attenuation makes it possible to keep the stretching of the metal within acceptable limits by a wide margin.

United States Patent 1151 3,638,597

Brown 1 1 Feb. 1, 1972 [54] METHOD OF FORMING A RIVET 3,479,979 11/1969Dunn ..113/121 R 3,366,086 111968 Fraze... 113/121 C [72] Bmwn Daywn'3,191,797 6/1965 Fraze... ..220/54 [73] Assignee: Ermal C. Fraze,Dayton, Ohio 3,479,733 1 1/1969 Brown.. 113/121 R 3,381,848 5/1968 Brown113/121 C [221 3,346,948 10/1967 Henning et al. .....29/509 [21] Appl.No: 866,064 3,307,737 3/1967 Harvey et al... .....220/54 3,545,24912/1970 Brown ..72/354 Related U.S. Appllcation Data [63]Continuation-impart of S61. No. 617,128, Feb. 20, 'f Y Lanham 1967, anda continuation-in-part of 704,766, Feb. 12, 45mm"! Emmme" Mlchael Keenan9 g Att0rneySmyth, Roston & Pavitt [52] US. Cl ..113/116 FF, 29/509,113/1 F, ABSTRACT 5 l t CI 1 13/ g gg A hollow rivet is formed in sheetmetal by first forming a dim- 15 A 6 H6 ple or hollow boss and thenconverting the dimple into the hol- 53 5? A 0 29/509 low rivet.Attenuation of sheet metal to form the dimple is car- R ried out byinitially stretching metal and subsequently squeezing the sloping wallof the dimple to extrude the sheet metal in opposite directions. Thesqueezing step to complete the at- [56] References cued tenuation makesit possible to keep the stretching of the metal UNITED STATES PATENTSwithin acceptable limits by a wide margin.

3,440,713 4/1969 Henchert ..29/509 72 Claims, 30 Drawing FiguresPATENTED FEB 1m 3.638.597

SHEET IBM 10 away/v2.

Omar Z. 5/010 nrmA/va J Pmmmrm m2 3538.597

SHEET OSUF i0 PATENTEDFEH um $638,597 SHEET lOUF 1O lA/VEA/TURJ 0074/ Z.6

Arm/M0547 METHOD OF FORMING A RIVET CROSS-REFERENCES TO RELATEDAPPLICATIONS This application is a Continuation-in-part of my copendingapplication entitled Method and Apparatus for Producing an IntegralRivet, Ser. No. 617,128, filed Feb. 20, 1967, and my copendingapplication entitled Method of Forming a Rivet, Ser. No. 704,766,filedFeb. 12, 1968.

BACKGROUND OF THE INVENTION In an easy-opening container, the containerwall is scored to form a tear strip or panel and a tab for manualseverance of the strip or panel is connected thereto by a hollow rivetthat is formed in the metal of the tear strip or panel itself. Thepresent invention relates in general to a method of forming such ahollow rivet as disclosed, for example, in the Fraze Pat. No. 3,191,564.That method is characterized by the concept of first forming a minorportion of the strip or panel into a gradually sloping boss or dimple insubstantially greater area in plan than the desired rivet andsubsequently reforming the dimple into the desired rivet configuration.

The broad object of the present invention is to meet certain needs forimprovement in this multiple-step method and especially to avoid certainlimitations that are encountered in prior art practices.

One limitation that is inherent in prior art methods of forming hollowrivets in sheet metal walls is that different dies must be employed forprocessing different thicknesses of sheet metal, for example to handlesheet metal varying from a given thickness by 0.003 of an inch plus orminus. The ideal would be the use of a single set of dies to processmany thicknesses of sheet metal in a given range. Such a set of dieswould drastically reduce the cost of the tooling as well as reduce thetime necessary for changing over from processing sheet metal of onethickness to sheet metal of another thickness.

Another limitation of the prior art procedures is that tooling designedfor successfully processing a given sheet metal hav ing particularcharacteristics may not prove acceptable when a sheet metal havingdifferent characteristics is substituted. It is to be borne in mind thatsheet metals employed for beverage can tops may be selected from a widerange of aluminum alloys, the alloys varying in hardness and ductility.The major portion of the cost involved in producing a beverage can ofthe easy-opening type is in the cost of the material and for this reasonthere is a trend towards using thinner sheet metal of relatively hightensile strength. The ideal process then, is a process having a widemargin of tolerance with respect to the properties of the sheetmaterial.

Another pressing need is to provide a rivet-forming technique that willmake it possible to locate the rivet close to the chuck wall of a canend thereby to provide a mechanical advantage in initiating theseverance of a tear strip or panel. Conventional dies for forming hollowrivets are of too large diameter to position a rivet close to a chuckwall and, moreover, if conventional dies are employed too close to achuck wall, the chuck wall will be deformed by the inherent tendency ofthe dies to pull the sheet metal radially inwardly into the region ofthe rivet. In this regard, the pressing need is for a rivet-formingprocess that may be carried out by tooling of small diameter with nosignificant tendency for the tooling to draw in the surrounding metal orto buckle or otherwise deform the surrounding metal.

Another need for improvement is to increase the service life of the diesand especially the dies that are employed to stake the rivets forpermanently fastening the tabs to the tear panels. Heretofore it hasbeen necessary to replace staking dies after relatively short periods ofuse.

Another pressing need is to reduce the pressure in tons that is requiredto fabricate an easy-opening can, the tonnage being high because anumber of different operations are carried out on each stroke of a pressincluding the operation of forming the hollow rivet and the operation ofstaking the hollow rivet into engagement with the tab. Usually thetonnage pressure required is so high that a relatively massive pressmust be used to carry out the progressive die operations simultaneously.

The problem of arriving at a method of fabrication that will meet all ofthese needs may be understood by first considering what kind of a hollowrivet is to be sought and by then considering how a preliminary dimplemay be formed that will make such a rivet possible.

The basic requirements for a hollow rivet are, first, that it be ofsufficient strength to withstand the loads in tension and shear that areimposed on the rivet by the operation of manually severing the tearstrip and, second, that the tubular wall of the rivet have sufficientresistance to axial compression to withstand the force of staking therivet without buckling or collapsing. These basic requirements have beenmet by prior rivets but the additional requirement to be met is that therivet be of a structure to reduce the pressure or impact force that isneeded to stake the rivet.

The energy required to stake a hollow rivet into permanent engagementwith the associated tab depends primarily on the height of the rivetrelative to the thickness of the tab. In a heretofore prevalentfabrication procedure the height of the rivet relative to the tab is toolow to permit the rivet to be staked by axial compression and thereforethe staking operation is necessarily carried out by applying high-impactforce across the thickness of the end wall of the hollow rivet to formthe necessary bead or head by radial extrusion of the end wall metal.For example, in the staking ofa hollow rivet with an initial end wallthickness on the order of 0.0135 of an inch, the thickness of the endwall may be drastically reduced to a residual of approximately 0.004 ofan inch. The difficulty is that when the end wall thickness is reducedto a residual less than 0.0045 of an inch, the impact force required onthe part of the staking punch rises drastically. The pressing need,therefore, is for a technique to increase the relative height of thehollow rivet so that the rivet head may be formed to a significantextent by bulging of the peripheral wall of the rivet under axialcompression and thus lessen the extent to which the staking operationdepends upon forcible thinning of the end wall. Reducing the impactforce has the further advantage of minimizing damage to the metal of theend wall as well as damage to the seal coat inside the end wall. Sincethe height of the rivet and the amount of metal in the rivet isdetermined by the preliminary dimple, this problem boils down to how toform a dimple of sufficient surface area to make a high rivet possible.An associated problem, of course, is how to reduce the die pressure thatis required to form the dimple since this die pressure is a major factorin the total tonnage pressure required for operating themultiple-station press for fabricating easy-opening can tops.

A dimple is formed by offsetting the sheet metal into a suitable diecavity and stretching the sheet metal to form the dimple must be keptwithin moderate bounds not only because excessive stretching undulywork-hardens the metal, but also because alloys used for can ends varywith respect to their tolerance for stretching and any rivet-formingmethod that requires immoderate stretching of the metal will beapplicable to only a limited range of sheet metal alloys. As heretoforenoted the trend is in the direction of thinner and harder metals forfabricating can tops and the thinner the metal and the harder the metalthe less its tolerance for stretching.

The necessity for stretching the sheet metal could be reduced by pullingsurrounding metal into the dimple area but, unfortunately drawingsurrounding metal into the dimple area must be minimized because itbuckles or otherwise deforms the surrounding metal and will distort thechuck wall or peripheral flange of a can end that is at all close to thedimple. Increasing the area of the flat metal that is offset to form thedimple would reduce the extent to which the sheet metal must bestretched to form the dimple, but this solution is ruled out for anumber of reasons. One reason is that the greater the dimple-formingarea the greater the tendency for surrounding metal to be drawn into thearea. Still another reason is that increasing the size of the dimplearea correspondingly increases the diameter of the dimpling dies withcorresponding intolerable increase in the minimum distance by which theultimate rivet must be spaced from the chuck wall of the can end. Forgood reasons, then, the flat metal that is to be used to form the dimplemust be of restricted area and with the limited area, the stretching ofthe metal within safe limits will not attenuate the metal enough toproduce a dimple of sufficient size to make a high rivet possible.

There is also a need for improvement that arises if the flat sheet metalaround the base of a dimple is coined to extrude metal into the dimplearea or if the flat area around a rivet is coined to cause radialiyinward metal flow to the rivet. Such a coining operation in thinning thesheet metal forms a step where the metal increases abruptly in thicknessat the outer edge of the coined area. In processing a can end to form atear strip, it is desirable to score the metal close to the rivet withthe line of scoring crossing the step in the thickness of the sheetmetal but to do so tends to fracture the metal in the region of thestep. This problem is made more acute by the use of harder and thinnersheet metals.

SUMMARY OF THE INVENTION A basic concept of the invention is that toproduce a dimple of suitably restricted diameter having the high ratioof surface area to plan area that is required for conversion of thedimple into a high rivet, the initially flat sheet metal may beattenuated in part by stretching and in further part by squeezing orcoining the metal of the sloping wall of the dimple to increase thesurface area of the dimple by extrusion. lt has been found that thesheet metal may be attenuated so effectively by adding the squeezingoperation, that attenuation by stretching may be kept within apermissible range for the whole spectrum of alloys that are suitable formetal containers.

A feature of the preferred practice of the invention is that thesqueezing operation is performed by the same punch that stretches thesheet metal. Thus in a single stroke of the press, the punch firststretches the sheet metal by offsetting the sheet metal into a diecavity and then the punch completes the attenuation of the sheet metalby squeezing the sheet metal against the wall of the die cavity as theend of the stroke is approached.

When the sheet metal is squeezed or coined in an annular zone by a punchthe displaced metal tends to flow in opposite directions away from thecoining zone and if the coining zone is outside of the die cavity, onlyone of the two directions of flow can increase the height of theresulting dimple. An important advantage of locating the coining zoneinside the die cavity on the slope of the shoulder of the dimple, isthat both the forward flow and the reverse flow may increase the heightand surface area of the dimple and for this purpose the dimpleformingdies may be designed to utilize the useful reverse flow.

An important advantage of the new dimple-forming method is that theforce required to thin metal in a plane that is at an acute angle to thedirection of die travel is less than the force required to thin metal ina plane that is perpendicular to the direction of die travel. The sameamount of work is performed, of course, but a lesser force acts througha greater distance. This fact may be understood when it is consideredthat the ratio between the rate of the thinning of the sheet metal andthe rate of travel of the punch is the sine of the angle of thecooperating die surfaces relative to the axis of the punch. For example,if the angle is 50 the sine is 0.766 and to reduce the thicknesses ofthe sheet metal by a given amount the punch must travel 1/0766 times thegiven amount or ap proximately 1.3 times the given amount. if the angleis 42 the ratio is 1.5 and if the angle is reduced to 30 the ratiobecomes 2.0. Thus at 42 the punch travel is 50 percent more than theextent to which the metal is thinned and at an angle of 30 the punchtravel is twice the amount that the sheet metal is thinned.

It is apparent that in comparison to the pressure that must be exertedby the press to thin metal in a plane that is perpendicular to thedirection of die travel, the pressure required in a plane at a 50 angleto the direction of die travel is reduced 24 percent and the pressurerequired in a plane at 30 to the direction of die travel is reduced by50 percent. in addition, the increased attenuation of the dimple wall bythe squeezing operation makes it easier to reform the dimple into therivet even though the rivet is higher than heretofore possible, therequired die pressure being reduced by more than 50 percent.

By virtue of the increased rivet height, the head of the rivet may beformed in part by simple axial compression of the rivet to reduce theextent that the end wall of the rivet must be thinned to form the rivethead. Thus the increased height of the rivet makes it possible to form arivet head of increased diameter and the staking operation may involvereducing the end wall of the rivet only to a residual thickness ofapproximately 0.0070.008 instead of a residual thickness of 0.004, thereduction in the staking die pressure being more than 50 percent. Therivet height may be increased to such extent as to make it unnecessaryto thin the transverse wall of the rivet head in the staking operations.

Since the operations of forming the dimple, forming the rivet andstaking the rivet account for the major portion of the pressure thatmust be exerted by the press, it may be understood how the new techniquemakes it possible to lower the total die pressures by more than 50percent. This reduction may permit a relatively light multiple station Cpress to be substituted for the usual heavy-duty multiple press having asliding ram head, the result being drastic reduction in capitalinvestment. The frame structure of a C-shaped press is inherently soresponsive to die pressures that if the total die pressure is immoderatesuch a press cannot be used for multiple station operation with a degreeof precision that is required for fabricating easy-opening can ends.

The concept of squeezing the sloping shoulder of the dimple to reducethe degree to which the sheet metal must be attenuated by stretching toproduce the desired increase in the surface area of the dimple, not onlymakes the method applicable to a wide range of alloys but also has theimportant advantage of making a single set of dimple-forming diescapable of processing different thicknesses of sheet metal stock. Allthat is necessary is to vary the shut height of the dimple dies inaccord with the thickness of the stock and for this purpose a set ofshims may be used to enable the dies to process sheet metal varying froma given thickness by as much as 0.003 of an inch plus or minus. Hereagain the acute angle of the plane of the squeezing action relative tothe direction of die travel is advantageous because adjusting the shutheight by a given distance causes the spacing between the cooperatingsqueezing surfaces of the die to vary by less than the given distance.Thus the inclination of the extrusion zone makes the die adjustment lesscritical and simplifies the changeover from one stock thickness toanother.

Given an attenuated dimple with a higher ratio of surface area to planarea than heretofore possible, the next step is to convert such a dimpleinto a high rivet. The conversion method set forth in theabove-mentioned Fraze patent is not suitable in this instance becausethe object of that method is to thicken the wall of the rivet andespecially the end wall whereas now the emphasis is to attain rivetheights. As will be explained, a new conversion method involvescorrelating two concurrent actions, one action being cooperation ofopposed planar die surfaces to flatten the dimple progressively in amanner to urge the dimple metal into the rivet die cavity, the otheraction being the action of a rivet punch to draw the sheet metal intothe die cavity. Thus the sheet metal of the dimple is pushed into thedie cavity by coacting planar die surfaces and at the same time ispulled into the die cavity by the punch to make possible a higher rivetthan heretofore attainable. The pushing action by the planar diesurfaces makes it possible to increase the stroke of the punch forincreasing the height of the rivet without consequent rupturing of thesheet metal by the punch.

Fortuitously the new fabrication procedure may be used to produce rivetssmaller in diameter than three thirty-seconds of an inch or if desired,rivets larger in diameter than one-quarter of an inch and the newfabrication procedure may be carried out in such manner as to avoidrupture of the sheet metal by a score line crossing a step at aperiphery of a coined zone.

The various features and advantages of the invention may be understoodfrom the following detailed description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, which are to beregarded as merely illustrative:

FIG. 1 is a top plan view of an easy-open can end utilizing a rivetconstructed in accordance with the teachings of this invention;

FIG. 2 is an enlarged fragmentary sectional view taken along line 2-2 ofFIG. I;

FIG. 3 is an enlarged fragmentary sectional view showing how the rivetsecures the tab to the tear strip of the container;

FIG. 4 is a fragmentary vertical sectional view through the dies used toform the dimple;

FIG. 5 is a vertical sectional view through a pair of reforming dies andthe dimple just prior to beginning ofthe reforming operation;

FIG. 6 is a view similar to FIG. 5 with the dies being illustrated atthe beginning of the reforming operation;

FIG. 7 is a view similar to FIG. 5 showing the dies after the reformingoperation has been completed;

FIG. 8 is a vertical sectional view through the staking dies before thestaking operation has begun;

FIG. 9 is a view similar to FIG. 8 with the staking dies being shown atthe completion ofthe staking operation;

FIG. 10 is a fragmentary sectional view through a second form of dieswhich are utilized to form a dimple having a central cap and an annularprotrusion;

FIG. 11 is a fragmentary sectional view through the dimple and thereforming dies which are utilized to reform the dimple into a hollowrivet;

FIG. 12 is a fragmentary sectional view through a third form ofdimple-forming dies;

FIG. 13 is a fragmentary sectional view through the dimple and a set ofdies which are utilized to reform the dimple into a hollow rivet;

FIG. 14 is a fragmentary sectional view similar to FIG. 13 after thedies have been moved together to convert the dimple into a hollow rivet;

FIG. 15 is a fragmentary sectional view through the hollow rivet, astaking punch, and a staking hammer prior to the time that the rivet isheaded;

FIG. 16 shows the dies illustrated in FIG. 15 after they have beenutilized to head the hollow rivet;

FIG. 17 is a fragmentary sectional view of a dimple and a fourth set ofdimple-forming dies to form the same;

FIG. 18 is a sectional view illustrating the tooling used to reform thedimple of FIG. 17 into a hollow rivet;

FIG. 19 is a sectional view through a dimple formed with the toolingshown in FIG. 17 with the dimple closely adjacent the chuck wall of thecan end;

FIG. 20 is a sectional view of a hollow rivet formed by the toolingshown in FIG. 18;

FIG. 21 is a fragmentary sectional view through an easyopening containerof the full panel pullout type, the tooling employed to form the rivetincluding the tooling shown in FIGS. 17 and 18;

FIG. 22 is a greatly enlarged sectional view of a dimple and a set ofdies to form the dimple;

FIG. 23, is a sectional view illustrating an initial stage in theconversion of the dimple of FIG. 22 into a rivet by a set of dies;

FIGS. 24-26 are sectional views like FIG. 23 showing further successivestages in the conversion ofthe dimple into a rivet;

FIG. 27 is a similar sectional view showing the completion of therivet-forming operation;

FIG. 28 is a sectional view illustrating the result of omitting the noseof the punch shown in FIGS. 23-27, the result being a rivet ofinadequate height;

FIG. 29 is a sectional view showing the step of staking a hollow rivetinto engagement with a pull tab; and

FIG. 30 is a view similar to FIG. 27 showing the completion oftheformation ofa rivet by a set of dies in which the punch is ofsubstantially greater axial dimension than the punch shown in FIGS.23-27.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION FIGS. l-9

Referring to FIGS. 1-3, reference numeral 15 designates an easy-openingcan end. The can end 15 has a peripheral flange 17 for attachment to acylindrical sidewall of a container (not shown). A score line 19 definesa generally teardrop-shaped tear strip 21 in the can end 15. A pluralityof beads 23 are formed integrally in the can end 15 to take up excessmaterial produced in the scoring operation and to make the can end 15more rigid. The tear strip 21 has a narrow leading end 25 positionedcentrally of the can end 15. A high rivet 27 is formed integrally withthe leading end 25 and extends through an aperture 29 in a tab 31 tosecure the tab to the tear strip.

The tab, which may be of the ring type illustrated or any other suitabletype, preferably has a hub 33 extending upwardly and surrounding theaperture 29. The rivet 27 as shown in FIGS. 2 and 3 has a generallycylindrical peripheral wall 35, a transverse end wall 37, and a bead 39.As best seen in FIG. 3, the hub 33 embraces the peripheral wall 35 andhas an upper end or shoulder 41 on which the circumferentially extendingbead 39 rests. A coined area 43 extends around the rivet 27 as shown inFIG. 3.

By lifting the end of the tab 31, stress is applied to the leading end25 of the tear strip 21 and to the score line 19 adjacent the leadingend of the tear strip. The rivet 27 and the leading end 25 of the tearstrip 21 is of very small area, and accordingly, the force applied bythe tab is highly concentrated along a relatively short length of thescore line 19 at the leading end to thereby substantially facilitateinitial severance thereof. The upwardly extending hub 33 provides asupport for the bead 39 and strengthens the tab to tear strip jointure.

A preferred method of making the hollow rivet 27 is illustrated in FIGS.4-9. The rivet-forming method consists of forming a minor portion of thecan end into a dimple or bubble (FIG. 4) and reforming the dimple (FIGS.5-7) into a hollow rivet. The resulting hollow rivet may then be stakedas shown in FIGS. 8 and 9.

In the dimple-forming operation shown in FIG. 4 a section of deformablesheet material 45 is first positioned on an upwardly directed exteriorface 47 of a female die 49. The female die has an arcuate surface 51that defines a die cavity which opens at the exterior face 47. A recess53 is formed axially in the arcuate surface 51. In the particularembodiment shown, the recess 53 is a passageway extending completelythrough the female die 49. However, if desired, the recess need onlyextend a portion of the way through the female die.

A male die or punch 55 having an arcuate working surface 57 cooperateswith the female die 49 to form a dimple 59 in the metal 45. The workingsurface 57 is adapted to be received within the die cavity formed by thearcuate surface 51 as shown in FIG. 5. With the working surface 57 soreceived in the die cavity, the arcuate surface 51 and the workingsurface 57 taper away from each other as they extend toward the exteriorface 47. Such tapering may be produced by making the arcuate surface 51and the working surface 57 portions of spheres with the radius of theformer being greater than the radius of the latter. It is preferred toflatten slightly a central end portion 61 of the working surface 57.

With the sheet metal 45 positioned on the exterior face 47 the punch 55is advanced toward the die cavity to offset a por tion of the sheetmetal 45 into the die cavity. No holddown means are provided for holdingthe portion of the sheet metal 45 immediately adjacent the die cavity incontact with the exterior face 47 during the dimple-forming operation.Because of the extruding of the metal as more fully describedhereinbelow, the sheet metal 45 moves upwardly during the dimple formingoperation to the position shown in phantom lines in FIG. 4.

The dimple 59 has a base portion 63, a thickened central region 65, anda sloping wall or wall section 67 extending therebetween. The wall 67progressively decreases in thickness as it extends toward the centralregion 65. The central region 65 is preferably circular in plan and hasan arcuate lateral wall 68. By way of example, when the sheet metal 45is initially 0.015 of an inch thick the base portion 63 may have athickness of 0.014 of an inch and an annular region 69 immediatelysurrounding the central region 65 may have a thickness of 0.009 of aninch. This progressive thinning of the wall 67 is caused by extrusion ofthe material of the wall 67 and results in a flow of metal along thesurfaces 51 and 57 to form the thickened central region 65 and toelongate the wall 67 thereby causing the sheet metal 45 to move upwardlyto the position shown in phantom line in FIG. 4 and increasing theheight of the dimple 59. The expression dimple height" is used hereinwhether the dimple is formed downwardly as shown in FIG. 4 or upwardlyas shown in FIG. 10.

Because the transverse end wall of the hollow rivet formed in theoperation shown in FIGS. -7 should be quite thick, the recess 53 (FIG.4) is provided to receive some of the extruded material of the wall 67so that the central region 65 will be substantially thicker than theannular region 69. For example, the central region 65 may have athickness of about 00135 of an inch when the sheet metal is originallyof 0.015 of an inch thickness.

The method described herein may be used with sheet material of variousthicknesses to produce dimples of various dimensions and the dimensionaldata stated herein is given solely by way of illustration, Thus, thediameter of the dimple 59 across the base 63 may be, for example, about0.406 of an inch and the diameter of the thickened central region 65 maybe, for example, about 0.085 of an inch. The die cavity in the femaledie 49 may have a depth of about 0.095 of an inch and the overall heightof the dimple as measured from the upper surface of the sheet metal 45(as viewed in FIG. 4) to the outer surface of the central region 65 maybe about 0.121 of an inch.

The dimple 59 is reformed by a male die 71 and a female die 73 shown inFIG. 5. An annular zone 75 surrounding the base portion 63 of the dimple59 is clamped between an annular clamping member 77 and an upper surface79 of the male die 71. This clamping arrangement prevents the metal inthe dimple 59 from flowing radially outwardly thereof or buckling duringthe reforming operation.

The male die 71 has a cylindrical member or punch 81 which extendsaxially into the dimple 59. As shown in FIG. 5, the height of the punch81 is less than the height of the dimple 59 and may be about 0.071 of aninch high. The diameter of the punch 81 is somewhat greater than thediameter of the central region 65 and may be about 0.103 of an inch.

The'female die 73 has a cylindrical opening 83 of sufficient diameter,e.g., 0.125 of an inch, to receive the punch 81 and an annular layer ofthe sheet metal of the dimple 59. The female die 73 has a lower face 85with an annular coining face 87 thereon immediately surrounding theopening 83. The female die 73 is adapted to move downwardly toward thedimpic 59 in a cylindrical passageway 89 formed in the clamping member77.

FIG. 6 illustrates the initial stages of the reforming operation. Thefemale die 73 moves downwardly to engage the wall 67 of the dimple S9and begins to deform the dimple in the manner illustrated. Therelatively thick portions of the wall 67 support thinner portions of thewall 67 adjacent the thickened central region 65 so that the thickenedcentral region 65 remains at substantially the same height throughoutthe reforming operation. The thinner portions of the wall 67 areinitially engaged by the die 73 and the metal thereof is deformedinwardly to begin the formation of the peripheral wall of a rivet 91(FIG. 7). At the completion of the reforming operation, theprogressively thinned wall 67 has been converted into a peripheral wall93 if the rivet 91 and the thickened central region 65 has beenconverted into a transverse end wall 95 of the rivet.

FIG. 7 shows the hollow rivet 91 at the completion of the reformingoperation. The peripheral wall 93 is annular and the transverse end wall95 is relatively thick and dome-shaped. The coining face 87 forms anannular coined zone 97 immediately surrounding the rivet 91. Because theannular zone 75 surrounding the coined zone 97 is clamped between theclamping member 77 and the male die 71 during the reforming operation,metal from the coined zone 97 is substantially prevented from flowingoutwardly and flows radially inwardly into the peripheral wall 93 of therivet 91. This causes the peripheral wall 93 to have substantialthickness even though the height of the rivet 91 is equal to orsubstantially equal to the height of the dimple 59. By way ofillustration, when the sheet metal is initially of 0.015 of an inchthickness, the peripheral wall 93 may have thickness of the coined zone97 and which may be approximately 0.009 of an inch.

The rivet formed by the operation described can have numerous uses ofwhich the can end shown in FIGS. 13 is illustrative. Similarly, therivet 91 may be staked according to any suitable procedure. However, itis preferred to stake the rivet 91 as illustrated in FIGS. 8 and 9.

1n the staking operation, a die member 99 having a cylindrical anvil 101is provided. The anvil 101 has an upper generally planar working face103. When the rivet 91 is placed over the anvil 101, the working face103 is preferably spaced substantially from the inner surface of thetransverse end wall 95 and is at a suitable height to preventinterference with the tab 31 or hub 33. The peripheral wall of the anvil101 is in engagement with or in substantial engagement with theperipheral wall 93 of the rivet. Next, the tab 31 is placed over therivet with the hub 33 embracing the lower portion of the peripheral wall93.

A hammer or punch 105 is positioned above the transverse end wall 95 ofthe rivet 91. The punch is forced downwardly into engagement with thetransverse end wall 95 to cause an initial folding of the peripheralwall 93 and partial axial collapse of the rivet 91. The folding of theperipheral wall initiates formation of a bead 107 (FIG. 9). The punch105 drives the transverse end wall 95 downwardly into engagement withthe working face 103 of the anvil 101 to cause the transverse end wallto be thinned or extruded between the punch and the working face 103.This causes the metal of the transverse end wall 95 to flow radiallyoutwardly to further assist in the formation of the bead.

FIGS. 10 and 11 FIGS. 10 and 11 illustrate a second embodiment of theinvention. Briefly, FIG. 10 shows a male die 121 and a female die 123being utilized to form a dimple or bubble 125. The dimple 125 includes acentral cap 127 surrounded by an annular protrusion 129. The central cap127 intersects the annular protrusion 129 to form an annular valley 131.It is apparent that the dimple 125 is very similar to the appearance ofthe dimple 59 as it appears during an intermediate stage of thereforming operation as shown in FIG. 6.

The wall of the annular protrusion 129 is thinner than the sheet metalblank from which the dimple is formed. The thinning through the wall ofthe annular protrusion 129 is the result of stretching of the metal bythe dies 121 and 123. By way of illustration, when the sheet metal stockis originally of 0.015 of an inch thickness the wall of the annularprotrusion 129 may be thinned to approximately 0.011 of an inch or 0.012of an inch.

The central cap 127 has a sloping progressively thinned wall section 135which extends from the valley 131 to a central protrusion 137 which isformed at a central region of the dimple 125. By way of illustration,the wall section 135 may be progressively thinned from about 0.01 l0.0l2of an inch adjacent the valley 131 to approximately 0.009 of an inchimmediately adjacent the protrusion 137. This thinning of the wallsection 135 is preferably brought about by extrusion of the materialthereof between the coacting dies 121 and 123. The extruded metal of thewall section 135 will flow toward the central region of the dimple 125and is utilized to produce the protrusion 137. The protrusion may have athickness of about 0.0135 of an inch and a diameter of about 0.085 of aninch.

Although all of the dimensions given herein are merely exemplary, it ispreferred to make the dimple 125 of relatively large diameter. Forexample, the diameter of the dimple may be approximately 0.7 of an inch,the distance between diametrically opposite points on the crest of theannular protrusion 129 may be about 0.48 of an inch and the distancebetween diametrically opposed points on the lowest region of the valley131 may be about 0.28 of an inch. Although the height of the dimple 125may also vary depending upon the desired final configuration of therivet, the overall height of the central cap 127 may be about 0.1 of aninch and the height of the annular protrusion 129 may be 0.080 of aninch.

As shown in FIG. 10, the dies 121 and 123 have working surfaces 139 and141, respectively, the contours of which generally conform to the finalshape of the dimple 125 except for annular recesses 141a and 141b. Thedie 121 includes an axial pin or punch 142 which is fixed relative to anannular portion 142a of the die 121. The working surfaces 139 and 141have tapered portions 145 and 147, respectively, which taper away fromeach other as they extend radially outwardly of the protrusion 137. Itis the tapering portions 145 and 147 which squeeze the wall section 135and cause extrusion of the material thereof to form the thickenedprotrusion 137 and the progressively thinned wall section. Unlike thedimple-forming operation of FIG. 4, this extrusion does not cause thesheet metal around the dimple 125 to move away from the supportingsurface. The die 123 has a recess or cavity 149 into which the extrudedmaterial from the wall section 135 may flow to form the protrusion 137.The extruded material from the wall section 135 also flows outwardly toassist in the formation of the annular valley 131 and the annularprotrusion 129. The recess 149 may take the form of a passageway whichextends all or a portion of the way through the die 123. The dies 121and 123 can be moved toward each other in any suitable controlledmanner. The die 123 may have a supporting body 151.

FIG. 11 shows the dimple 125 resting on a male die 153 with a female die155 disposed thereabove. The male die 153 is substantially identical tothe male die 71 (FIG. 6) and has a broad fiat supporting surface 157 anda working surface or punch 159 which extends upwardly into the dimple125.

The die 155 is similar to the die 73 shown in FIG. 5. Thus, the die 155has a passageway or cavity 161 therein of sufficient diameter and depthto receive the punch 159 and the resultant hollow rivet. The die 155also has a coining face 163 which causes flow of metal inwardly into thewalls of the rivet to further thicken them. The dies 153 and 155 aremovable toward each other in the same manner shown in FIGS. 6 and 7 toproduce a hollow rivet as shown in FIG. 7. The height of the resultingrivet is about equal to the height of the dimple 125 and the resultingrivet may be staked as illustrated in FIGS. 8 and 9.

FIGS. 12-16 These figures illustrate a third embodiment of a preferredpractice of this invention. FIG. 12 shows how a male die or punch I65and a female die 167 can be utilized to form a dimple 169 which has aprogressively thinned sloping wall or wall section 171 and a centralregion 173 which is no thicker than the surrounding portions of the wall171. Generally, the dimple 169 is identical to the dimple 59 (FIGS. 4and except that the former has no thickened central region and issomewhat higher than the dimple 59. In the formation of the dimple 169,the thickened central region is eliminated and the material that wouldhave been utilized to form such thickened central region is utilized toprovide additional elongation of the wall 171 and hence additionaldimple height. The sheet metal 175 surrounding the dimple 169 isunrestrained so that it can move away from the exterior face 177 to theposition shown in phantom in FIG. 12 to allow for elongation of the wall171. Illustrative dimensions of the rivet 169 are the same as thosegiven in connection with a dimple 59 except that the central region 173would have a thickness of only about 0.009 ofan inch and the height ofthe dimple 169 is greater.

The punch has a working face 179 and is generally identical to the maledie 55. The female die 167 is identical to the female die 49 except thatthe former has no recess for thickening of the central region 173. Thefemale die 167 has a cavity forming an arcuate surface 181. The workingsurface 179 and the arcuate surface 181 taper away from each other asdescribed in connection with the first embodiment and act to squeeze thewall 171 to extrude the material thereof to elongate or provideincreased height to the dimple 169. Accordingly, the dimple 169 may besomewhat higher than a corresponding dimple formed in accordance withthe teachings of the first embodiment ofthe invention.

As shown in FIGS. 13 and 14, the dimple 169 is reformed into a hollowrivet 185 by a female die 187 and a punch 189. The punch 189 has aworking surface 191 extending axially into the dimple 169 and the femaledie 187 has a coining face 193 for causing flow of metal radiallyinwardly into the wall of the rivet 181. During the reforming operationand the coining operation, the sheet metal is clamped between twomembers 195 and 197 which retard or substantially prevent outward flowof metal during the coining operation. The female die 187 and the punch189 function in the same manner described above in connection with thedie 73 and the punch 71 to reform the dimple 169 into a hollow rivet ofaheight approximately equal to the height of the dimple 169. The hollowrivet 185 is identical to the hollow rivet 91 shown in FIG. 8 exceptthat the former has a constant wall thickness which may be approximately0.009 of an inch throughout and also is somewhat higher than the rivet91.

It is preferred to stake the rivet 185 without substantially thinning adome-shaped transverse end wall 199 thereof. Generally, optimum resultsare achieved when the transverse end wall 199 is thinned approximately0.001 of an inch to 0.003 of an inch to a final thickness ofapproximately 0.0060.008 of an inch, the preferred final thickness being0007-0008 of an inch. FIGS. 15 and 16 illustrate one way in which thiscan be accomplished. The sheet metal 175 is supported by a surface 201and a stepped staking punch 203 which is mounted in a stepped recess 204is biased upwardly by a coil spring 205. The staking punch 203 entersthe hollow rivet 185 and is spaced from the interior surface of thetransverse end wall 199 and may form a sliding fit with the interiorsurface of a peripheral wall 207 of the rivet. A punch or hammer 209 isutilized to strike the outer surface of the transverse end wall 199 topartially axially collapse the rivet 185. The interior surface of thetransverse end wall 199 is thus moved into engagement with the stakingpunch 203 to drive the latter downwardly as the rivet 185 is furthercollapsed. Preferably, the punch 203 is driven downwardly until itcontacts a lower wall 211 of the recess 204. At this instant the punch209 is at, or substantially at, the bottom of its stroke and at thisinstant the transverse end wall 199 is thinned about 0.001 of an inch to0.003 of an inch, preferably 0.00l-0.002 of an inch. The punch 203 alsoserves to prevent radial inward collapse of the peripheral wall duringthe staking operation. The rivet 185 firmly secures a tab 213 having ahub 215 to the tear strip.

It is apparent that the operation illustrated by FIGS. 15 and 16 hasthree stages. In the first stage the initial lowering movement of theupper punch 209 tends to flatten the domed end wall of cavity, rivetagainst the stationary lower punch 203 with consequent initiation of thebead-forming operation by radially outward bulging of the end of therivet.

In the second stage the transverse end wall of the rivet is clampedbetween the two punches under spring pressure and the two punches movedownward in unison to carry the end wall downward with consequentcollapse of the rivet axially. With the lower or inner punch 185preventing inward collapse, the collapse of the rivet is outward. Thusthe formation of the head that is continued in the second stage iscarried out by outward folding the peripheral wall of the rivet onitself, the folding action occurring because the leading lower face ofthe upper punch 209 is large enough in area to overhang the radiallyexpanding end wall of the rivet throughout the second stage.

In the third stage the lower punch 203 stops and the upper punch 209continues downward a short distance to squeeze the transverse end wallof the rivet against the lower punch for slight thinning of the endwall. For example a transverse end wall of the rivet of an initialthickness of 0.009 of an inch may be reduced by only 0001-0002 of aninch since only slight additional radial expansion of the end wall ofthe rivet is required to complete the circumferential bead of the rivetin view of the fact that the first two stages of the operation expandthe end wall so effectively.

FIGS. 17-21 In this fourth embodiment of the invention, FIG. 17 shows apiece of deformable sheet material 311 such as aluminum sheet metalbeing offset by a bubble punch 313 and a bubble die 315 to form a bubbleor dimple 317. The die 315 has a conical die surface 319 at leastpartially defining a die cavity 321 and extending between an exteriorface 323 of the die and a central recess or bore 325 which alsopartially defines the die cavity 321. The die 315 has a chamfer 327intermediate the die surface 319 and the exterior face 323 tosubstantially prevent drawing of any of the sheet material 311 into thedie cavity 32] from an area of the sheet material lying radiallyoutwardly of the dimple 317. By way of example, the chamfer 327 may forman included angle with the exterior face 323 of about 145 and be blendedinto the inner die surface 319 and the face 323 with relatively smallradii to form relatively sharp corners to thereby prevent this radialinward flow of material during the dimple-forming operation.

The punch 313 is retained within a suitable holder 329 and has afrustoconical lateral working surface 331 and a rounded nose 333 whichblends into the conical working surface 331. The punch 313 and the die315 are suitably arranged to be axially advanced toward each other inany suitable conventional manner. In the embodiment illustrated in FIG.17, the dimple 317 is formed downwardly to better illustrate the coiningof the dimple will; however, it should be understood that the dimple 317may be formed upwardly, if desired.

To form a dimple 317, the sheet material 31 1, which may be assumed tobe in a substantially flat condition, is positioned between the punch313 and the die 315. The punch 313 and the die 315 are then relativelyaxially advanced toward each other to cause the nose 333 of the punch313 to engage the sheet material and deform a portion thereof downwardlyinto the die cavity 321 to initiate formation of the dimple 317. Thus,in the initial phase of the dimple-forming operation, the sheet materialforced into the die cavity 321 is stretched and therefore made thinnerin accordance with conventional dimple forming methods. However, due tothe chamfer 327 and the adjacent relatively sharp corners, substantiallyno material is drawn radially inwardly into the dimple 317 from an areasurrounding the dimple.

The step of relatively advancing the punch 313 and the die 315 towardeach other is continued to allow the wall 335 of the dimple 317 to besqueezed between the die surface 319 and the working surface 331 todisplace material of the dimple wall along such surface to therebyincrease the axial dimension of the dimple. Such squeezing or extrusionof the material of the dimple wall 335 causes flow of the material inboth directions along the surfaces 319 and 331. The flow of materialtoward a base 337 of the rivet 313 may operate to elevate thesurrounding portions of the sheet material 311 above its originalposition, the original position of the sheet material being illustratedin phantom lines in FIG. 17. Such elevation is made possible by notclamping the sheet material around the dimple 317. The flow of materialtoward a central region 339 may move such central region axially awayfrom the nose 333 of the punch 313 to form a gap or space 341. Thesqueezing or coining of the dimple wall 35 is very important inproducing a dimple having substantial height, i.e., a substantial axialdimension, without fracturing or excessively thinning the dimple wall.Without such coining step, excessive thinning or fracture of the dimplewall would occur and the central region 339 of the dimple would bepunched out by the nose portion of the punch before the desired dimpleheight was reached.

The dimple formed with the punch 313 and the die 315 is shown moreclearly in FIG. 19. FIG. 19 illustrates the dimple in an invertedposition from that shown in FIG. 17 and as having been formed from thesheet material 311 of a can end. As the dimple forming method shown inFIG. 17 substantially prevents drawing of sheet material from an areasurrounding the base 337 of the rivet 313 into the dimple wall 335, thedimple 317 may be formed closely adjacent a chuck wall 343 of the canend without distorting the latter. In the form shown in FIG. 19, it isimportant that such drawing action be substantially prevented in thearea of the sheet material lying intermediate the dimple 317 and theclosely adjacent portion of the chuck wall 343 illustrated. The chuckwall 343 terminates upwardly in the usual peripheral curl or flange 345for attaching the can end to a body of a container (not shown). Therelatively small diameter of the punch 313 and the die 315 alsocontribute to the formation of the dimple 317 closely adjacent the chuckwall 343.

The importance of coining of the dimple wall 335 and in particular theimportance of the conical surfaces 319 and 331 can best be illustratedby reference to a specific example. Assuming the sheet material 311 tohave an original thickness of about 0.010 of an inch, the dimple 317 mayhave an axial dimension measured from the underside of the sheetmaterial 311 to the upper surface of the central region 339 ofapproximately 0.090 of an inch, and a diameter across the base 337 ofonly approximately 0.250 of an inch with the diameter of the recess 315being about five thirty-seconds of an inch. The dimple wall 335 may havea thickness of about 0.007 to 0.008 of an inch. A dimple having suchsubstantial height and wall thickness for such a relatively smalldiameter could not be produced without coining of the dimple wall 335.The use of a conical die surface 319 and a conical lateral workingsurface 331 is also preferred because of the relatively steep anglewhich the dimple wall 335 may assume relative to the sheet material 311.Although the specific angle in the embodiment illustrated is notcritical, such angle may be about 57. A steep angle has the advantage oftending to produce a relatively high dimple for a given diameter acrossthe base 337.

Of course, the particular dimensions given above are stated solely byway of example and can be varied over a wide range depending upon theresults desired. If the coining of the dimple wall 335 is allowed toprogress sufficiently, the central region 339 may be somewhat thickerthan the surrounding regions bf the dimple wall 335. The portion of thedimple wall 335 that is squeezed between the die surface 319 and theworking surface 331 is of accurately controlled thickness and shape, andin the embodiment shown such portion is the region of the dimple wall335 adjacent the base 337. Preferably. the dimple-forming operation iscarried out so that the dimple is slightly thicker adjacent the base 337than adjacent the central region 399. This may be accomplished, forexample, by utilizing slightly different angles on the die surface 319and the working surface 331 so that these surfaces are slightly inclinedtoward each other as they extend toward the central region 339. A dimplewith a base which is thicker than the region of the dimple wallimmediately surrounding the central region can be more easily reformedinto a hollow rivet.

Thus, with the present invention the dimple is initially formed by anoffsetting procedure during which the material of the dimple isstretched. Ultimately, the dimple wall 335 is forced into the die cavityto the extent that it can be squeezed between the die surface 319 andthe working surface 331 to extrude some of the material of the dimplewall as described above. Both of these stretching and coining functionscan be carried out with a single punch and die as shown in FIG. 17;however, these functions could be carried out with different punches anddies, if desired.

The dimple 317 is reformed by a button punch 347 and a button die 349into a hollow rivet 351. As best seen in FIG. 20 where the rivet 351 hasbeen inverted, the rivet has a peripheral wall 353, a transverse endwall 355, and a base 357. The button die 349 has an interior die surface359, defining a die cavity or recess 361 and opening at an annularworking surface 363 which surrounds the recess. The button punch 347 hasa head portion 365 which is receivable within the recess 361 and anannular working surface 367 surrounding the head portion. The outerannular regions of the annular working surfaces 363 and 367 shouldpreferably taper away from each other and this is accomplished in theembodiment illustrated by the die 349 which has an annular inclined ortapered surface 369 which extends away from the annular working surface367 as it extends radially outwardly of the rivet 351.

To reform the dimple 317, the dimple is positioned between the punch 347and the die 349 and then these tools are axially advanced toward eachother. The dimple 317 and the rivet 351 can be formed upwardly ordownwardly or in any other orientation although it is preferred not toinvert the dimple in preparation for the reforming step. The annularworking surface 363 of the die 349 contacts a peripheral region of thedimple wall 335 and collapses or flattens the latter toward the annularworking surface 367 of the punch 347. The peripheral region of thedimple wall so collapsed, is converted into a circumscribing zone 371which surrounds the base 357 of the rivet 351 as may be seen in FIG. 2.An inner region of the dimple is converted into the rivet 351. The headportion 365 of the punch 347 is within the dimple 317 to thereby controlthe ultimate shape of the rivet 351. The circumscribing zone 371 isinherently of lesser thickness than the material of an area 373 lyingradially outwardly of the circumscribing zone. This is so because thecircumscribing zone 371 was originally formed from a peripheral regionof the dimple wall 335 which has been thinned by stretching and coiningas described above in connection with FIG. 17. To eliminate any abruptchange of thickness of the sheet material intermediate thecircumscribing zone 371 and the area 373, the punch 347 and the die 349are relatively advanced to allow the annular working surfaces 363 and367 to squeeze or coin very slightly the sheet material of thecircumscribing zone 371 and of an inner annular region of the area 373.Specifically, the parallel portions of the working surfaces 363 and 367engage the sheet material of the circumscribing zone 371 to iron out thematerial thereof to assure that this region will at least besubstantially fiat. The material of the circumscribing zone 371 may beextruded or coined only very slightly to iron out these irregularitieswithout significantly or materially increasing the axial dimension ofthe rivet 351 or without increasing the thickness of the wall 351 and353 of the rivet. The annular tapered surface 369 of the die 349cooperates with the outer annular regions of the working surface 367 toform a tapered transition section 375 of gradually increasing thicknessas it extends radially outwardly to thereby prevent any substantialchange in thickness between the circumscribing zone 371 and the area373. Thus, when a line of weakness such as a score line 377 (FIG. isformed through the area 373 and the circumscribing zone 371, as is oftenthe case in easy-opening containers, such score line will not extendthrough any abrupt change of thickness of the sheet material. Thisironing or slight coining around the base of the rivet is useful withany rivet-forming method which leaves abrupt changes of thickness in thesheet material adjacent the rivet.

Although the rivet 351 may be used in many different environments, it isparticularly adapted for use with an easyopening container as shown byway of example in FIG. 21. FIG. 21 illustrates a can end 379 of thefull-panel pullout type having a chuck wall 381 and a line of score 383in the can end defining a panel 385 at least partially removabletherefrom and covering substantially all of the can end. A rivet 387constructed in accordance with the teachings of this invention is formedintegrally with the can end 379 and lies very close to the score line383 and the chuck wall 381. The rivet 387 secures a tab 389 of suitableconstruction to the can end. The tab 389 has a nose end 391 whichoverlies the score line 383 and which bears downwardly against the scoreline upon lifting of the opposite end of the tab to thereby initiaterupture of the material of the can end along the score line 383 in aconventional manner. The tab 389, when lifted upwardly pivots about ahinge line which preferably extends intermediate the rivet and theadjacent portion of the chuck wall 389 in a conventional manner. As therivet is located very close to the score line 383 and to the chuck wall381 as permitted by the features of the present invention, the length ofthe tab from the hinge line to the nose end 391 can be materiallyreduced to significantly increase the mechanical advantage of the tab389.

FIGS. 22-30 FIG. 22 illustrates a method of forming a dimple 400 in asheet 402 of deformable materials by means of a punch, generallydesignated 404, and a cooperating die, generally designated 405. The die405 has a cavity 406 with a transverse surface 408 of the die extendingin all directions from the rim of the cavity. The rim of the cavity isformed with a chamfer 409, the surface of which is at an angle of 30relative to the transverse surface 408. The punch 404 has a nose portion410 and has a transverse surface 411 that extends in all directions fromthe root or base of the nose portion 410. The two transverse surfaces408 and 411 are spaced substantially apart to keep from confining thesheet 402 in the regions surrounding the cavity.

When the nose portion 410 initially advances into the die cavity 406 itstretches the sheet material to the configuration of the nose portionand thus forms what may be termed a preliminary dimple. After thepreliminary dimple is formed by stretching the metal in this manner, anannular surface of the leading end of the punch cooperates with acorresponding annular surface of the die cavity 406 to squeeze thepreliminary dimple across the thickness of its wall in an annular zonethat is designated A in FIG. 22. The die surfaces that cooperate in thesqueezing operation are preferably at least in part of sphericallycurved configuration and both surfaces are inclined in general at anacute angle to the axis of the punch.

Just what portion of the dimple is converted into a rivet itself andwhat portion is converted into the adjacent portion of the containerwall that surrounds the rivet depends upon the shape and dimensions ofthe rivet forming dies. In general, however, the annular zone A" in FIG.22 becomes a part of the container wall surrounding the rivet and atleast a major part of the remaining central portion of the dimple thatis designated 8" in FIG. 22 is converted into the rivet itself.

The squeezing of the sheet material across its thickness in the annularzone A" causes the sheet material to flow or extrude towards the centralzone 8" with consequent bulging of the central zone or peak portion ofthe dimple away from the nose of the punch 410. Thus an importantadvantage of squeezing the sloping wall of the dimple is that it occurswhen the punch approaches maximum penetration into the die cavity and inthe absence of the squeezing action would abruptly increase the tensionof the sheet material with possible rupture of the sheet material. Withthe sheet material squeezed to such extent as to cause the sheetmaterial to bulge in advance of the punch, however, the squeezingoperation actually completely eliminates tensioning of the material ofthe dimple as the maximum offsetting of the sheet material isapproached.

It is to be noted that the two dies shown in FIG. 22 provide an annularspace 412 surrounding the nose portion 410, this annular space providingample freedom for the squeezing action on the dimple to cause reverseflow or extrusion of the material in addition to the forward flowtowards the peak of the dimple. The reverse flow of the materialincreases the height of the dimple and causes the surrounding sheet 402to rise somewhat above the transverse die surface 408 as indicated bythe gap 413. It is also to be noted that the edge of the rim formed bythe chamfer 409 substantially prevents the punch from drawing the sheetmaterial into the die cavity during the stretching stage of the dieoperation and thus prevents distortion of the surrounding portion of thesheet 402 to make it possible to locate the dimple close to the chuckwall of a can top. The chamfer is equivalent to a radius but has lesstendency to tear the sheet material.

The described method of forming a dimple is highly ad'- vantageous inthe manner in which it distributes the material of the dimple inpreparation for the rivet-forming operation. The displacement of sheetmaterial from the annular zone A" into the central zone 8" where therivet is to be formed increases the mass of the rivet with consequentincrease in the strength of the rivet. The described method of formingthe dimple is also advantageous in that the peak or crown of thedimple'400 is relatively thick, being thicker than the rest of thedimple and this relatively thick peak portion of the dimple is largelyutilized for the end wall of the rivet. Using some of the increased massof the rivet to thicken the end wall of the rivet reduces the diepressure that is necessary to stake the rivet into engagement with apull tab and using some of the increased mass to increase the height ofthe rivet also reduces the required staking pressure.

A dimple produced in the heretofore conventional manner substantiallysolely by stretching the sheet material has a given area in plan and agiven ratio of surface area to area in plan and is capable of producinga rivet of a given size and strength. Referring again to FIG. 22, thenew method of fabricating the dimple has three results, namely, increasein the ratio of surface area to the plan area of the dimple; increase inthe mass of the part of the dimple that ultimately forms the rivet; andthe improved distribution of that mass to increase the strength of therivet. With these three results in mind, it may be readily understoodthat the gains achieved by the new dimple-forming technique may be usedin various ways.

The new technique may be applied to a dimple of the abovementioned givenarea in plan to produce a rivet of greater size than the previouslymentioned given rivet size, i.e., a rivet that is of greater heightand/or greater diameter from a dimple of the above-mentioned given area.Or the new dimple-forming method may be utilized to produce a rivet ofthe previously mentioned given rivet size from a dimple of less than thepreviously mentioned given area in plan and thus make it possibie tolocate the rivet closer to the chuck wall of a can top.

Or the new technique with the resulting more efficient distribution ofthe mass of the rivet may be utilized to produce a strong rivet of lessdiameter than the diameter of the previously mentioned rivet of a givensize from a dimple of substantially less area in plan than thepreviously mentioned given area in plan. Thus the new technique makes itpossible to reduce the diameter of a hollow rivet in a beverage can topto a relatively small diameter in the range of less than threethirty-seconds of an inch and further makes it possible to locate such arivet relatively close to the chuck wall of the can top.

As heretofore noted, the fact that the coacting squeezing surfaces ofthe dies in FIG. 22 are inclined at an acute angle to the axis of thepunch results in reduction of the die pressure that is required to thinthe wall of the dimple to a given degree, the rate of advance of thepunch being low relative to the rate of displacement of the sheetmaterial by the squeezing action.

It is contemplated that the set of dies shown in FIG. 22 will be capableof forming dimples in a selected list of sheet materials that varywidely in ductility, i.e., vary widely in the percentage of stretch ofthe sheet material that may be tolerated without damaging the sheetmaterial. For this purpose the set of dies shown in FIG. 22 is designedto stretch the sheet material within the minimum tolerance to stretchfound in the selected list of sheet materials. The stretch tolerance ofthe sheet material no longer limits the height of the rivet. Thisfeature of the invention makes it possible to employ a single set ofdimpleforming dies successfully for a number of widely difierent sheetmetal alloys in the selected list.

Heretofore it has been customary to design a set of dimpleforming diesto handle sheet material of a particular thickness, it being necessaryto provide a number of different sets of dies to handle sheet materialof a number of different thicknesses. In this regard, a feature of theset of dies shown in FIG. 22 is it may be used to process sheets of awide range of different thicknesses, for example the range of thicknessthat might vary 0,003 plus or minus. The degree to which the set of diesenlarges a preliminary dimple configuration by the squeezing actiondepends upon the extent to which the punch penetrates the wall of thepreliminary dimple. Thus to form a dimple in relatively thin sheetmaterial the degree to which the punch approaches the cavity surface isincreased and to make a dimple in thicker sheet material the extent towhich the punch ap proaches the cavity surface is reduced.

In practice it is a simple matter to employ interchangeable shims toback up one of the two dies of the set so that to change from processingsheet material of one thickness to processing sheet material of adifferent thickness it is merely necessary to substitute one shim foranother. In this regard it is to be noted that by virtue of theinclination of the coacting die surfaces, a given amount of axialadjustment of the coacting dies changes the extent to which the metal issqueezed by less than the given amount. In other words, the inclinationof the coacting die surfaces reduces the criticalness of the dieadjustment in addition to reducing the required die force or pressure.

FIGS. 23-27 show successive stages in the conversion of the dimple 400of FIG. 22 into the hollow rivet 414 in FIG. 27 by a set of diescomprising a lower die 415 with a cylindrical cavity 416 and an upperpunch 418. The lower die 415 has a transverse surface 420 that extendsgenerally radially from the rim of the cavity 416 and this transversesurface includes an annular plateau 422 that surrounds the cavity and isformed with an annular sloping shoulder 424. The upper punch 418 has anose portion 425 that is dimensioned to fit into the die cavity 416 withclearance for the wall of the rivet and the upper die has a transversesurface 426 that extends radially from the root or base of the noseportion.

At the start of the rivet-forming operation the dimple 400 seats on therim of the die cavity 416 as shown in FIG. 23 with the sheet 402parallel with the transverse surface 426 of the punch 418. In FIG. 24illustrating an early stage in the closing movement of the two dies, thenose portion 425 of the punch is pushing the peak portion of the dimpleinto the die cavity 416 and the rim of the die cavity forms an annularshoulder 428 in the sloping wall of the dimple. In FIG. 25 where thenose portion is further advanced into the die cavity 416, a somewhatwider annular shoulder 430 is formed in the sloping wall of the dimple.

Throughout the rivet-forming operation from the start shown in FIG. 23to the completed stage shown in FIG. 27, a pushing force created by thecooperation by the horizontai die surfaces 420 and 426 cooperates with apulling force by the nose portion 425 that places the sheet materialunder tension as the sheet material progressively enters the die cavity416. In effect the pushing force funnels the material of the dimple intothe die cavity and the nose 426 acting against the peak portion of thedimple pulls the sloping circumferential wall of the dimple into the diecavity. The pushing and pulling forces must be correlated to avoid, onthe one hand, tearing of the material by the punch and on the other handto avoid wrinkling of the sheet material.

The manner in which the pushing force funnels the material of the dimpleof the die cavity 4I6 may be understood by considering the forcesinvolved. In FIG. 24 the arrow 432 represents a force which is thetremendous resistance to radial outward flow of the metal in the planeof the flat sheet 402 that surrounds the base of the dimple. The arrow434 represents the closing force of the dies, i.e., the downwardmovement of the upper horizontal die surface 426 towards the lowerhorizontal die surface 420. The arrow 435 is the resultant of the twoforces and it is to be noted that the inclination of the arrow at leastapproximates the inclination of the sloping wall of the dimple tominimize any tendency of the sloping wall to be buckled or folded by theresultant force.

FIG. 25 shows the same inclined force 435. A vertical arrow 436represents the resistance to downward movement of the material that isoffered by the lower die surface 420. The resultant of forces 435 and436 is the horizontal radially inward force represented by the arrow 438which drives the sheet material towards the brink of the die cavity 416.Since initially the dimple is bowed into the die cavity as shown in FIG.23 and since the dimple is increasingly bowed into the die cavity, theconsequence of the radially inward force 438 is a pronounced tendencyfor the material of the dimple to curl inwardly into the die cavity.

The fact that the pushing force tends to funnel the material of thedimple into the die cavity 416 may be demonstrated in the manner shownin FIG. 28 where a plain metal block 439 with a lower planar workingsurface 440 is substituted for the punch 418. When a dimple 400 isinitially positioned on the lower die 415 in the manner shown in FIG. 23and the dies shown in FIG. 28 are progressively closed, the result ofthe funneling of the material of the dimple into the die cavity 416 isthe formation of an inadequate hollow rivet 442 which is too low inheight to serve the purpose of attaching a pull tab to a tear strip. Itis apparent that to the extent that the cooperating transverse surfaces426 and 420 of the dies shown in FIGS. 23-27 tend to form the hollowrivet 442 shown in FIG. 28, the burden placed on the nose portion 425 ofthe punch 418 to stretch the sheet material is correspondingly reduced.

With this explanation of the mode of operation of the rivetforming diesshown in FIGS. 23-27, a person skilled in the art can readily design aneffective die set empirically by simply starting with a die set thatwould seem to be capable of obtaining the desired result and thenchanging the shapes and dimensions of the die parts to arrive at a finaldesign by cut and try. A good procedure is first to arrive at a dimpleconfiguration and a lower die configuration which will produce theresult shown in FIG. 28 in the absence of the upper punch. Once theresult shown in FIG. 28 is obtained a punch with a nose portiondimensioned in accord with the die cavity size will produce the desiredrivet.

As heretofore explained, the squeezing of the material of a dimple in anannular zone A creates an abrupt shoulder or change in the thickness ofthe sheet material at the base of the dimple and since, as indicated inFIG. 27, the area becomes the portion of the container wall thatsurrounds the base of the dimple, the abrupt change in thickness of thesheet material of the dimple at the outer circumference of the annulararea A" produces an abrupt annular shoulder in the sheet material, thelocation of the annular shoulder being indicated at 444 in FIG. 23. Thefunction of the sloping annular shoulder 424 of the lower die 415 is toreduce the abruptness of the annular shoulder in the manner heretoforeexplained. Thus at the completion of the rivet-forming operation theannular wall of the container surrounding the base of the rivet has agradual change in thickness that is determined by the inclination of theannular die shoulder 424.

The finished rivet shown at 414 in FIG. 27 has an axial dimension orheight of 0.080 of an inch which is determined by the axial dimension445 of the nose portion 425 of the punch and which is greater than theaxial dimension of a conventional rivet formed in the conventionalmanner. FIG. 29 shows how the rivet 414 shown in FIG. 27 may be stakedinto engagement with a surrounding pull tab 448 of an easy-opening cantop. The lower die 450 in FIG. 29 has an anvil portion 452 that nestsinto the underside of the rivet to give support to the circumferentialwall 454 of the rivet within the planes that are defined by the tab 448and the can top 402. The upper die has a punch 455 of substantiallygreater diameter than the rivet which cooperates with the lower die tostake the rivet, i.e., to form an annular bead 456 in overhangingengagement with the rim of the circular opening 458 in the pull tab.

The increased height of the rivet 416 that results from the processillustrated by FIGS. 22-27 greatly reduces the downward force requiredby the punch 454 to stake the rivet. As heretofore stated the increasedheight of the rivet makes it possible to form the bead 456 in part byoutward bulging and collapse of the circumferential wall of the rivetand obviously less force is required to form a head in part by suchfolding of the metal than is required to form the bead solely bysqueezing the end wall 460 of the rivet to extrude the end wall materialradially outwardly. The staking operation does squeeze the end wall ofthe rivet for radially outward extrusion of the end wall material butonly a relatively small amount of metal need be extruded and it takesless force to create the required amount of radial extrusion in arelatively thick end wall than in a relatively thin end wall. If sheetmetal of a starting thickness of 0.0135 of an inch is reduced by thestaking operation to the usual residual thickness of 0.004 the diepressure rises tremendously at the end of the die stroke. The end wall460 of the rivet in FIG. 29 has a liberal residual thickness of at least0.007 and a further advantage is that the outside diameter of the bead456 is 0.156 of an inch instead of the usual 0.150 of an inch.

A special advantage of the described rivet-forming operation is that thedies shown in FIGS. 23-27 may be used for sheet metal varying from agiven thickness by 0.003 of an inch plus or minus with no change otherthan change in the shut height to which the dies close.

Because of the helpful funneling action on the dimple of the transversesurfaces of the rivet-forming dies as exemplified by FIG. 28, the axialdimension of the nose portion of the upper punch of the rivet-formingdies may be even greater than shown in FIG. 27 to produce a rivet ofeven greater height than the rivet 414. FIG. 30, for example shows anose portion 464 of an axial dimension 465 that is greater than theaxial dimension 445 to result in a rivet 466 of a height ofapproximately 0.1 ID of an inch. Increasing the height of the rivet inthis manner while still producinga rivet of adequate strength evenfurther reduces the amount of the die pressure that is required in thestaking operation.

The importance of the invention has been emphasized with reference tothe dies and punch presses that may be employed for mass production ofthe end walls of easy-opening containers. It is also to be emphasizedthat the invention minimizes the causes of damage to the seal coat onthe inner surface of the sheet metal. Exposure of a minute area of themetal can spoil the flavor ofa beverage and for this reason it isconventional practice to postseal" the can ends, i.e., apply additionalseal coating on the inner coated side of the can ends along the scorelines and inside the staked hollow rivets. It is a simple matter topostcoat the scored areas but postcoating the inner surfaces of thestaked heads of the rivets is difficult and is a problem in high-speedproduction.

The most important feature of the invention for minimizing damage to theseal coat is the reduction in the pressure of the dies across thethickness of the sheet material in the step of staking the hollow rivet.As heretofore pointed out the usual exponential rise in pressure acrossthe end wall of the rivet as the reduction in thickness of the end wallapproaches 0.004 of an inch is avoided and the necessary pressure isreduced 50 percent by increasing the height of the rivet so that thestaking or radial expansion of the bead of the rivet may be accomplishedlargely by collapsing the circumferential wall of the rivet instead oflargely if not solely by radial extrusion of the metal of the end wall.

The staking procedure illustrated by FIGS. 15 and 16 is especiallyadvantageous in converting the sealing efficiency of

1. In a method of making a hollow rivet, the steps of: providing asection of deformable sheet material; deforming a portion of saidsection of sheet material to form a bubble having a sloping wall sectionof progressively substantially decreasing thickness as said wall sectionextends toward a central region of the bubble; and reforming said bubbleinto a hollow rivet having a peripheral wall and a transverse end wallby converting said central region into at least a portion of saidtransverse end wall of the rivet and by converting at least a portion ofsaid sloping wall section into at least a portion of said peripheralwall of the rivet whereby the relatively thick portion of said slopingwall section of said bubble supports the thinner portion of said slopingwall section during the reforming of the bubble to thereby give therivet substantial height.
 2. A method as defined in claim 1 wherein saidstep of reforming includes displacing the material from an annular zoneimmediately surrounding said rivet inwardly into the peripheral wall ofsaid hollow rivet.
 3. In a method of making a hollow rivet, the stepsof: providing a member of deformable sheet material; offsetting aportion of said member to form a dimple having a base, a central region,and a wall surrounding said central region extending toward said base;progressively thinning at least a portion of said wall betweencooperating die members to cause the material of said portion of saidwall to flow so that said wall is relatively thin adjacent the centralregion of the dimple and is progressively thickened as it extendsoutwardly thereof; and reforming said dimple into a hollow rivet havinga peripheral wall and an end wall.
 4. A method as defined in claim 3wherein said step of reforming includes converting said central regioninto at least a portion of said end wall of the hollow rivet, convertingat least a portion of said wall of said dimple into at least a portionof said peripheral wall of the rivet, and displacing a portion of thedeformable material around the base of the rivet into said peripheralwall of the rivet.
 5. A method as defined in claim 3 wherein saidcentral region is not thicker than the thickness of the portion of thewall of the dimple immediately thereadjacent.
 6. In a method of making ahollow rivet, the steps of: providing a member of deformable material;offsetting a portion of said member to form a dimple having a base, acentral region, and a wall section surrounding said central region andextending outwardly thereof; squeezing said wall section betweencooperating die members to cause at least a portion of the material ofsaid wall section to flow toward the central region to increase thethickness thereof so that the central region is thicker than the portionof the wall section immediately thereadjacent; and reforming said dimpleinto a hollow rivet having a peripheral wall and a relatively thicktransverse end wall by converting the central region of the dimple intoat least a portion of the transverse end wall and by converting at leasta portion of the wall section of the dimple into at least a portion ofsaid peripheral wall.
 7. A method as defined in claim 6 wherein thematerial that flows toward the central region forms a generally axiallyextending protruding projection at the central region.
 8. In a method offorming a hollow rivet in a member of deformable sheet maTerial, thesteps of: deforming a section of said member to form a dimple having abase, a central region, and a wall section surrounding said centralregion and extending away from said central region toward said base,said wall section being of minimum thickness adjacent the central regionand progressively increasing in thickness as it extends away from saidcentral region, said central region having an inner surface spaced apredetermined distance from the plane of said member; and reforming saiddimple into a hollow rivet having a peripheral wall and a transverse endwall with the inner surface of the transverse end wall being at least atsaid predetermined distance from the plane of said member whilepreventing said peripheral wall and said transverse end wall from havingthicknesses materially less than said minimum thickness.
 9. In a die setfor forming a hollow rivet in a member of deformable material, thecombination of: a female die having an exterior face and a slopingsurface, said sloping surface defining a die cavity extending apredetermined distance into said female die and opening at said exteriorface; and a cooperating male die having a sloping working surfaceadapted to be at least partially received within said die cavity, themember of deformable material being positionable between said die cavityand said working surface, said working surface being movable toward saidmember to offset a portion of the member into said female die to form adimple having a sloping wall between said sloping surface and saidworking surface, the working surface of said male die and the slopingsurface of said female die tapering away from each other as they extendtoward said exterior face of said female die when said working face isat least partially received within said recess whereby continuedmovement of said working surface causes progressive thinning of thesloping wall.
 10. In a method of making a hollow rivet, the steps of:providing a section of deformable sheet metal of predeterminedthickness; deforming a portion of said section of sheet metal to form abubble having a base and a sloping wall surrounding a central region ofthe bubble; causing the sheet metal at said central region of the bubbleto be of greater thickness than the portion of said sloping wall sectionimmediately surrounding the central region by extruding at least some ofthe metal of the sloping wall so that the sloping wall is thinneradjacent the central region than adjacent the base and to cause aportion of the metal of the sloping wall to flow toward the centralregion; and reforming said bubble into a hollow rivet having aperipheral wall and a transverse end wall by converting said centralregion into at least a portion of said transverse end wall of the rivetwhereby the transverse end wall is of substantial thickness and byconverting at least a portion of said sloping wall section into at leasta portion of the peripheral wall of the rivet.
 11. In a method of makinga hollow rivet from deformable sheet material, the steps of: offsettinga portion of the sheet material to form a dimple having a dimple wall, abase and a central region with the dimple wall protruding outwardly froma surrounding zone of sheet material; squeezing at least a section ofthe dimple wall while the dimple wall protrudes outwardly from thesurrounding zone to displace at least some of the material of saidsection to thereby influence the configuration of the dimple; andreforming the dimple into a hollow rivet.
 12. A method as defined inclaim 11 wherein said step of squeezing is carried out by die membersand each of said die members has a generally spherical portion forsqueezing said section of the dimple wall whereby said dimple isgenerally bowl-shaped and has a general spherical portion.
 13. A methodas defined in claim 11 wherein said dimple has a central cap surroundedby an annular protrusion, said central cap containing a central regionof the dimple and said section of the dimple wall with the latterextending outwardly from said central region toward the annularprotrusion to an area adjacent said annular protrusion.
 14. A method asdefined in claim 11 wherein said step of squeezing includes displacingthe material of the dimple wall so that the dimple wall is thinneradjacent the central region of the dimple that adjacent the base of thedimple.
 15. A method as defined in claim 11 wherein said step ofreforming is carried out subsequent to said step of squeezing.
 16. Amethod as defined in claim 11 wherein said section of the dimple walllies intermediate the base and the central region of the dimple.
 17. Ina method of making a hollow rivet from deformable sheet material, thesteps of: providing a die having a surface defining a die cavity and apunch having a working surface; relatively advancing said punch and saiddie toward each other to forcibly engage the sheet material and force aportion of the sheet material into the die cavity to form a dimplehaving a dimple wall and a central region; continuing said step ofrelatively advancing to squeeze at least a section of the dimple wallbetween said surfaces to extrude some of the material thereof toward thecentral region of the dimple; and reforming the dimple into a hollowrivet.
 18. A method as defined in claim 17 wherein the squeezing of thesection elongates the dimple wall and increases the axial dimension ofthe dimple and including the step of allowing a zone of the sheetmaterial surrounding said die cavity to move away from said die duringthe squeezing of the dimple wall, whereby the increase in the axialdimension of the dimple forces the material of said zone away from thedie.
 19. A method as defined in claim 7 wherein said surface of said diehas a recess therein disposed generally axially within said die cavityand the squeezing of said dimple wall causes a portion of the materialof the dimple wall to flow into the recess to form an outwardlyextending projection on said dimple, said projection being thicker thanthe portions of said wall section immediately thereadjacent.
 20. Amethod as defined in claim 17 wherein said surface of the die and saidworking surface taper away from each other when said punch is advancedinto said die cavity whereby the squeezing of said dimple wall betweensaid surface of the die and said working surface progressively thinssaid dimple wall so that the dimple wall is relatively thin adjacent thecentral region and progressively increases in thickness as it extendsaway from said central region.
 21. In a method of securing a tab havingan aperture therethrough to a container wall of sheet material, thesteps of: deforming a portion of the container wall to form a bubblehaving sloping wall section and a central region; forcibly engaging atleast a portion of the sloping wall section between cooperating diesurfaces to extrude at least some of the material thereof along said diesurfaces toward the central region, without deforming said portion intothe plane of the container wall; reforming said bubble into a hollowrivet; forming a line of weakness in the container wall adjacent thehollow rivet to define a segment of the container wall at leastpartially removable therefrom; placing the tab over the hollow rivetwith the rivet extending through the aperture; and heading the rivet tosecurely clamp the tab to the container wall.
 22. A method as defined inclaim 21 wherein said step of forcibly engaging displaces the materialof the wall section so that said wall section is of maximum thicknessadjacent the base thereof and of lesser thickness adjacent a centralregion of the dimple.
 23. In a method of making a hollow rivet fromdeformable sheet material the steps of: offsetting a portion of thesheet material to form a dimple having a central region and a slopingdimple wall which slopes with respeCt to a region of the sheet materialsurrounding the dimple; applying a compressive force to at least asection of said sloping dimple wall with such force being sufficient tocause flow of at least some of the material of said section generallytoward the central region of the dimple; said step of applying beingcarried out with said section of the sloping dimple wall slopingrelative to said region which surrounds the dimple and without deformingsaid section of the sloping dimple wall into the plane of saidsurrounding region whereby said section of the dimple wall continues toform a portion of the dimple during said step of applying; and reformingthe dimple into a hollow rivet.
 24. A method as defined in claim 23wherein said compressive force is applied by coacting die surfaces eachof which slopes relative to said region of sheet material surroundingthe dimple.
 25. In a method of making a high dimple from sheet materialwithout fracturing the sheet material, the steps of: providing a diehaving a die surface defining a die cavity and a punch having a slopingworking surface at least partially receivable in the die cavity;relatively advancing said punch and die to offset a portion of the sheetmaterial into said die cavity to form a dimple having a base, a centralregion and a sloping wall with the dimple being surrounded by asurrounding zone of the sheet material which lies outside of said diecavity and with the sloping dimple wall protruding outwardly from saidsurrounding zone; forcibly engaging at least a section of said slopingdimple wall between coacting compression surfaces to apply a compressiveforce to said section with said compressive force being sufficient tocause flow of at least some of the material of said section along saidcompression surfaces to cause said sloping dimple wall to assume apredetermined configuration; said step of forcibly engaging beingcarried out with said section of the sloping dimple wall slopingrelative to said surrounding zone and without deforming said section ofthe sloping dimple wall into the plane of said surrounding regionwhereby said section of the sloping dimple wall is surrounded by saidsurrounding zone of sheet material.
 26. A method as defined in claim 25wherein said step of relatively advancing includes relatively advancingsaid punch and die a predetermined amount, said compression surfacesincluding said working surface and said die surfaces, and said step offorcibly engaging includes continuing said step of relatively advancingto squeeze the dimple wall between said working surface and said diesurface to apply said compressive force to said section of said slopingwall.
 27. A method as defined in claim 25 wherein said step of forciblyengaging displaces the material of the sloping dimple wall so that saidsloping dimple wall is of maximum thickness adjacent the base thereofand of lesser thickness adjacent the central region of the dimple.
 28. Amethod as defined in claim 25 wherein said surrounding region is notrigidly restrained immediately adjacent said die cavity and said step offorcibly engaging causes at least some of the material of said sectionof said sloping wall to flow toward the base of the dimple to therebymove the base of the dimple away from said die cavity whereby the axialdimension of the dimple is increased.
 29. In a method of making a hollowrivet of deformable sheet material wherein the sheet material in acircumscribing zone at least partially surrounding the rivet is thinnedas a result of the formation of the rivet and the sheet material in anarea outside of the circumscribing zone and adjacent thereto is thickerthan the sheet material of the circumscribing zone thereby resulting ina relatively abrupt change of thickness of the sheet material betweenthe area and the circumscribing zone, the improvement to preventfracture of the sheet material as a result of scoring through the areaand the circumscribing zone compriSing the step of: squeezing the sheetmaterial of an inner region of said area adjacent the circumscribingzone between coacting die members to displace at least some of the sheetmaterial of said inner region to gradually reduce the thickness of saidinner region as it extends toward the rivet without materiallyincreasing the axial dimension of the rivet and to convert said innerregion into a tapered transition section joining the circumscribing zoneto the outer region of the area without any abrupt changes of thicknessof the sheet material.
 30. A method as defined in claim 29 wherein eachof said die members has an outer annular region with the outer annularregions of said die members being generally confronting and taperingaway from each other as they extend radially outwardly, said outerannular regions of the die members engaging the inner region of saidarea during said step of squeezing, each of said die members alsoincluding cooperating surface portions for engaging the circumscribingzone therebetween during said step of squeezing to smooth out the sheetmaterial of the circumscribing zone to prevent any abrupt change ofthickness therein.
 31. In a method of making a hollow rivet fromdeformable sheet material, the steps of: forming a dimple in the sheetmaterial having a dimple wall of lesser thickness than the originalthickness of the sheet material and with the sheet material of an areasurrounding the dimple wall being of substantially said originalthickness; reforming the dimple to convert the dimple into a hollowrivet having lesser area in plan than said dimple and to convert aperipheral region of the dimple wall into a circumscribing zone at leastpartially surrounding the hollow rivet with such circumscribing zone ofthe dimple wall having a lesser thickness than the original thickness ofthe sheet material; and displacing some of the sheet material from saidarea adjacent the circumscribing zone to gradually reduce the thicknessof the sheet material in a region of said area adjacent thecircumscribing zone to thereby taper said region of said area to avoidany abrupt change in thickness of the sheet material between said areasof the circumscribing zone as a result of formation of the rivet.
 32. Amethod as defined in claim 31 wherein said step of displacing includessqueezing the sheet material of the circumscribing zone between coactingdie members to iron out the sheet material so squeezed to therebyfurther guard against abrupt changes in thickness of the sheet materialas a result of formation of the rivet.
 33. In a method of fabricating aneasy-opening container in which a container wall of deformable materialis scored along a line defining a tear strip and a tab is attached tothe tear strip by a hollow rivet formed in the tear strip and whereinthe hollow rivet is formed by first forming in the container wall adimple and then a central portion of the dimple is converted into therivet with the remaining outer annular portion of the dimple forming awall portion adjacent the rivet that is crossed by the line of scoring,the improvement comprising: forming the dimple by attenuating thecontainer wall in part by stretching the wall material and in part bysqueezing the wall material across its thickness with consequentformation of a shoulder in the region of said outer annular portion ofthe dimple where the thickness of the wall material is changed abruptlyby the squeezing operation; forming said central portion of the dimpleinto the rivet and forming the outer annular portion of the dimple intosaid wall portion adjacent the rivet in one operation and in the sameoperation subjecting said wall portion to pressure across its thicknessto reduce the abruptness of said shoulder thereby to permit the wallportion to be scored across the shoulder without rupture of the wallmaterial at the shoulder.
 34. An improvement as set forth in claim 33 inwhich a punch is employed to offset a portion of the container wall intoa die cavity to form the dimple and the punch cooperates with a surfaceof the die cavity to squeeze the wall material and the squeezing actionis carried out with sufficient attenuation of the wall material to causethe wall material to bulge away from the nose of the punch.
 35. A methodof forming a dimple of a desired configuration in a sheet of deformablematerial characterized by the steps of: placing the sheet materialacross a die cavity; advancing a punch against the sheet material tooffset the sheet material into the die cavity with consequentattenuation of the sheet material by stretching; and continuing theadvance of the punch relative to the cavity to cause an annular portionof the punch to squeeze the dimple wall across its thickness against thesurface of the cavity for further attenuation of the sheet material, thesqueezing action extruding the sheet material to the extent of causingthe dimple to bulge beyond the nose of the punch to increase the area ofthe dimple.
 36. In means to form a dimple in a sheet of deformablematerial wherein the dimple is formed by a punch making a stroke tooffset the sheet material laterally of its plane into a die cavity. theimprovement to reduce the tendency of the punch to damage the sheetmaterial by excessively stretching the sheet material, comprising: thepunch and the die cavity having respective surfaces at acute angles tothe axis of the punch formed to cooperate to squeeze the sheet materialprogressively in response to approach of the punch to the end of thestroke to cause extrusion of the sheet material in the direction of thepunch travel and thus curtail stretching of the sheet material by thepunch.
 37. In a method of forming a hollow rivet in a sheet ofdeformable material by first forming a minor portion of the sheet into adimple in the sheet and then reforming the dimple to the desired rivetconfiguration, wherein the amount of material for the rivet provided bya dimple of a given area in plan depends on the ratio of the surfacearea to the plan area of the dimple and if the dimple is formed bystretching the sheet material within the tolerance of the material adimple of a given area in plan with a given area ratio is required toform a rivet of adequate strength of a given size, the improvement toincrease the area ratio of the dimple to make possible the formation ofa rivet of adequate strength of said given size by means of a dimple ofless than the given area in plan or to make possible the formation of arivet of adequate strength of larger than said given size by means of adimple of said given area in plan, comprising: attenuating said minorportion of the sheet by stretching the minor portion within thetolerance of the sheet material; and additionally attenuating the minorportion of the sheet by squeezing the sheet across its thickness in azone of the minor portion to extrude the sheet material out of the zone.38. An improvement as set forth in claim 37 in which the stretching stepis carried out by first forming an initial dimple of a particular arearatio and then the squeezing step is carried out to convert the initialdimple into a dimple of a higher area ratio.
 39. An improvement as setforth in claim 38 in which the wall of the initial dimple slopes at anacute angle to the axis of the dimple and the squeezing step is carriedout at least in part in an annular zone of the sloping wall.
 40. Animprovement as set forth in claim 38 in which the initial dimple isformed by offsetting a portion of the sheet into a die cavity; and inwhich the squeezing step is carried out by a punch positioned inside thepreliminary dimple to squeeze the material of the preliminary dimpleagainst the inner surface of a die cavity.
 41. An improvement as setforth in claim 38 in which the initial dimple is formed by advancing apunch into a die cavity to stretch the sheet by pushing a portioN of thesheet laterally into the die cavity; and in which the squeezing step iscarried out by continuing to advance the punch into the die cavity tosqueeze the wall of the dimple between the surface of the punch and thesurface of the cavity.
 42. An improvement as set forth in claim 41 inwhich the squeezing step is carried out by squeezing an annular portionof the preliminary dimple against an annular portion of the surface ofthe cavity.
 43. An improvement as set forth in claim 42 in which thesqueezing step is carried out both to extrude the sheet material towardsthe peak of the dimple and to extrude the sheet material in the oppositedirection out of the die cavity to increase the height of the dimple bytwo opposite directions of extrusion.
 44. An improvement as set forth inclaim 37 in which the squeezing step is applied to a portion of thedimple to provide a pattern of thickness variation across the crosssection of the dimple that results in a pattern of thickness across thecross section of the rivet to make the rivet stronger than a rivet ofthe same size that is produced without the squeezing operation.
 45. Aset of dies to form dimples or the like in sheets of various deformablematerials, the tolerance of the various materials to stretch lyingwithin a given range, comprising: a die with a cavity therein; and apunch shaped and dimensioned to force a portion of the sheet materialinto the cavity to stretch said portion to form an initial dimple andthen to cooperate with the die by squeezing the wall of the dimpleagainst the surface of the cavity to cause flow of the wall material toincrease the size of the initial dimple, said die and punch being shapedand dimensioned to form an initial dimple of a size that requiresstretching of the sheet material within the minimum tolerance of saidrange so that the die and punch may be used to form dimples in all ofsaid various deformable sheet materials.
 46. In the fabrication ofeasy-opening containers having pull tabs attached to tear panels byhollow rivets formed in the tear panels, a method of forming dimples insheets of deformable material for subsequent conversion into the hollowrivets, the thicknesses of said sheets lying within a given range ofthicknesses, characterized by the steps of: providing a die with acavity therein having an annular portion with a surface at an acuteangle relative to the axis of the cavity; providing a cooperative punchdimensioned to enter the cavity and force the sheet material into thecavity to form a preliminary dimple, said punch having an annularportion to cooperate with said annular portion of the die cavity tosqueeze the wall of the preliminary dimple across its thickness forextrusion of the wall material to increase the size of the dimple; andvarying the extent to which the die and punch approach each other byincreasing the extent when a changeover is made from sheet material of agiven thickness to sheet material of a lesser thickness and decreasingthe extent when a changeover is made from sheet material of a giventhickness to sheet material of greater thickness, whereby the single setof die and punch may be used for all sheets of said given range ofthicknesses.
 47. In the operation of a multiple-station press forprogressively fabricating easy-opening can ends having pull tabsattached to tear panels by hollow rivets formed in the tear panels,wherein dimples of a given ratio of surface area to plan areas areconventionally formed in the can ends substantially solely by stretchingthe material of the can ends within the tolerance of the sheet materialat one of the press stations, the dimples are conventionally reformedinto hollow rivets of a given size by a set of dies at another stationof the press, and the hollow rivets are staked into engagement with thetabs at still another station of the press, the improvement to lower thetotal of the die pressures at the statioNs of the press thereby topermit a relatively light press to be used at a relatively lowinvestment cost, comprising: providing a punch at said one station toforce the sheet material into the cavity to stretch the sheet materialto form a preliminary dimple therein and then to squeeze the wall of thepreliminary dimple against the surface of the cavity to cause extrusionflow of the sheet material to convert the preliminary dimple into alarger final dimple, said punch and die cavity having cooperativesqueezing surfaces inclined at acute angles to the axis of the punch tocause the rate of the squeezing of the sheet material to be low relativeto the rate of die travel and thus require relatively low die pressures.48. An improvement as set forth in claim 47 in which the squeezing stepis carried out to give the final dimple a ratio of surface area to planarea greater than said given ratio thereby to increase the materialavailable for forming the hollow rivet; and which includes providing aset of dies at said another station of the press to take advantage ofthe increase in available material by forming rivets higher than rivetsof said given size with consequent reduction in the pressure required tostake the rivets at said still another station.
 49. An improvement asset forth in claim 47 in which the final dimple has a peak portionsurrounded by an annular sloping wall; in which said another station ofthe press is equipped with a die having a cavity of the cross section ofthe desired rivet with a surface extending laterally in all directionsfrom the rim of the cavity; and in which said another station of thepress is further equipped with a punch having a nose portion with asurface extending laterally in all directions from the base of the noseportion, said cavity and said lateral surfaces being shaped anddimensioned relative to the dimple to force material of the dimple intothe cavity with a funneling action in the absence of the nose portion tosuch extent as to form a shortened hollow rivet in the cavity in suchabsence, whereby the rivet is formed in part by pushing of the materialinto the cavity by cooperation of said lateral surfaces and the rivet isformed in further part by the nose portion of the punch acting on saidpeak portion of the dimple to pull said annular sloping wall into thecavity.
 50. In an apparatus for fabricating a hollow rivet in a sheet ofdeformable material wherein a first set of dies forms a dimple of agiven configuration in the sheet and a second set of dies including adie with a cavity therein together with a punch cooperative with thecavity reforms the dimple into the hollow rivet, the improvementcomprising: said die of the second set of dies being shaped anddimensioned to cause the dimple to be converted into a hollow rivet ofinadequate axial dimension in the die cavity when a block of larger areathan the cavity is substituted for the punch, whereby the punch isrelieved of a portion of the burden of forcing the material of thedimple into the die cavity thereby to reduce the stretching of thematerial by the punch and to make possible the formation of a rivet ofincreased axial dimension.
 51. In a method of forming a hollow rivet ina sheet of deformable material wherein a dimple is formed in the sheetand then the dimple is reformed to utilize a central portion of thedimple to form the rivet and to utilize the surrounding annular portionadjacent the base of the dimple to form the wall that extends radiallyfrom the base of the rivet, the improvement to increase the strength ofthe rivet, comprising: squeezing said annular portion of the dimpleacross the thickness of the dimple wall prior to the reforming step todisplace material from the annular portion into the central portion toincrease the mass of material in the subsequently formed rivet.
 52. In amethod of forming a hollow staked rivet in a sheet of deformablematerial to aTtach an apertured sheet metal member to the sheet whereinan initial step is to form a dimple in the sheet, a subsequent step isto reform the dimple into a hollow rivet and a still later step is astaking step to form a circumferential bead around the head of the rivetin engagement with said sheet metal member, the staking step including afinal operation of thinning the end wall of the rivet by squeezing theend wall across its thickness, the improvement over conventionalrivet-forming methods for the purpose of reducing the degree to whichthe end wall of the hollow rivet is reduced in thickness by the stakingoperation and for the further purpose of reducing the damage to any sealcoat that may be present on the inner surface of the rivet, comprising:increasing the surface area of the dimple relative to its area in planin comparison to the surface areas of conventional dimples relative totheir areas in plan by attenuating the sheet material to form the dimplenot only by deforming the sheet material in a conventional manner butalso by squeezing the sheet material across its thickness to increasethe height of the dimple; utilizing the consequent increased size of thedimple to form a hollow rivet of greater height relative to the area incross section of its midsection than the height of conventional rivetsrelative to the areas in cross section of their midsections; placing theapertured sheet metal member on the hollow rivet; and then staking therivet of increased height to provide it with an overhangingcircumferential bead by first collapsing the rivet to the height of aconventional rivet, thereby taking advantage of its exceptional heightto form the major part of the desired bead and then only slightlythinning the transverse end wall of the rivet by squeezing the end wallacross its thickness to complete the bead.
 53. An improvement as setforth in claim 52 which includes the step of blocking bulging of thematerial of the rivet beyond the plane of the end wall during thecollapsing of the rivet.
 54. An improvement as set forth in claim 52which includes the step of internally reinforcing the rivet throughoutsubstantially the full length of the circumferential wall of the rivetand throughout substantially the whole collapsing step to preventradially inward collapse of the circumferential wall of the rivet. 55.An improvement as set forth in claim 52 in which the step of axiallycollapsing the rivet is carried out by first clamping the end wall ofthe rivet across its thickness under pressure and, while still clampingthe end wall, forcibly moving it axially of the rivet towards the baseof the rivet.
 56. An improvement as set forth in claim 55 in which thethinning of the end wall is accomplished by increasing the clampingpressure on the end wall after the rivet is collapsed.
 57. Animprovement as set forth in claim 52 in which the step of forming thedimple is carried out by closing a male die and a cooperating female diethat has a cavity to receive the formed dimple: and in which the twodies respectively have surfaces cooperative to squeeze the sheetmaterial for extrusion of the sheet material to increase the surfacearea of the dimple relative to its plan area.
 58. An improvement as setforth in claim 52 in which the step of staking the rivet includes:placing a first die inside the rivet against the inner surface of theend wall thereof; placing a second die against the outer surface of theend wall of the rivet; urging the two dies toward each other to clampthe end wall of the rivet under pressure across its thickness; movingthe two dies simultaneously toward the base of the rivet to collapse therivet to a desired height by moving the clamped end wall towards thebase of the rivet; and then moving one of the two dies towards the otherof the two dies to thin the end wall of the rivet.
 59. A method ofstaking a hollow rivet of deformable sheet material to provide the Rivetwith an overhanging circumferential bead, including the steps of:placing a first die inside the rivet against the inner surface of theend wall thereof; placing a second die against the outer surface of theend wall of the rivet; urging the two dies toward each other to clampthe end wall of the rivet under pressure across its thickness; movingthe two dies simultaneously toward the base of the rivet to shorten therivet by collapsing the rivet to a desired height by moving the clampedend wall towards the base of the rivet; and then moving one of the twodies towards the other of the two dies to thin the end wall of therivet.
 60. A method of staking a hollow rivet of deformable sheetmaterial to provide the rivet with an overhanging circumferential bead,including the steps of: placing a first die outside of the rivet inposition confronting the outer surface of the end wall of the rivet;placing a second retractable die inside the rivet in extended positionconfronting the inner surface of the end wall of the rivet, with thesecond die urged by spring pressure to its extended position; advancingthe first die towards the second die to cooperate with the second die toclamp the end wall under pressure and to retract the clamped end wall inopposition to spring pressure to collapse the rivet to a selected heightand thereby to form the major part of the desired bead; and thenstopping the retraction of the second die and continuing the advance ofthe first die to squeeze and thin the end wall to complete the saidbead.
 61. A method as set forth in claim 58 in which the second die isdimensioned to prevent inward collapse of the rivet.
 62. A method as setforth in claim 58 in which the leading face of the first die overhangsthe end wall of the rivet to prevent the material of the rivet frombulging past the plane of said leading face.
 63. In the processing of ahollow rivet having circumferential wall and a transverse end wall,means to stake the hollow rivet by forming a continuous bead around thecircumference of the rivet, comprising: a first die having a leadingface to contact the outer surface of the end wall of the hollow rivet; asecond retractable die to advance into the hollow rivet into contactwith the inner surface of the end wall of the rivet; a secondretractable die to advance into the hollow rivet into contact with theinner surface of the end wall of the rivet; spring means to urge thesecond die to advance; means to advance the first die a predetermineddistance to cause the first die to cooperate with the second die toclamp the end wall of the rivet under the pressure of the spring meansand to retract the clamped end wall to collapse the rivet, thereby toform the major portion of said bead; and means to stop the retraction ofthe second die before the advance of the first die is completed, therebyto cause the first die to cooperate with the second die to thin the endwall of the rivet thereby to complete said bead.
 64. Means to stake ahollow rivet as set forth in clamp 63 in which the second die isdimensioned to prevent radially inward collapse of the rivet.
 65. Meansto stake a hollow rivet as set forth in claim 64 in which the leadingface of the first die overhangs the end wall of the rivet to prevent thematerial of the rivet from bulging past the plane of said leading face.66. In the processing of a hollow rivet having circumferential wall anda transverse end wall, means to stake the hollow rivet by forming acontinuous bead around the circumference of the rivet, comprising: twodies to clamp the end wall of the rivet under pressure; means to movethe two dies simultaneously to retract the end wall towards the base ofthe rivet to collapse the rivet to a predetermined height and therebyform the major portion of said bead; and means to increase the pressureof the two dies on the end wall of the rivet when the rivet iS collapsedthereby to thin the end wall to complete said bead.
 67. In an apparatusto form a dimple in sheet material in preparation for forming a hollowrivet in the sheet material, the combination of: first die meansincluding a die with a cavity; second die means including a punch toforce the sheet metal into the cavity for the purpose of forming thedimple, the punch and the die cavity having respective surfaces formedto cooperate to squeeze the sheet material in response to approach ofthe punch to the end of its stroke to cause extrusion of the sheetmaterial and thus curtail stretching of the sheet material by the punch;and means provided by at least one of the two die means to substantiallyprevent the punch from drawing the sheet material into the die cavitythereby to substantially prevent distortion of the sheet materialadjacent to the die cavity.
 68. A combination as set forth in claim 67in which the means to substantially prevent the punch from drawing thesheet material into the cavity comprises a chamfer around the rim of thecavity.
 69. A combination as set forth in claim 67 in which the die withthe cavity therein has a surface surrounding the cavity that is at anangle of 90* relative to the axis of the punch; and in which the meansto substantially prevent the punch from drawing the sheet material tothe die cavity comprises a chamfer around the rim of the cavity, thechamfer being at an angle of substantially 30* relative to the saidsurrounding surface.
 70. In an apparatus to form a hollow rivet in asheet of deformable material, the combination of: first die meansincluding a pair of coactuating dies to form a dimple in the sheet, oneof said dies having a cavity, the other of the dies being a punch toenter the cavity with consequent initial stretching of the sheet, thepunch and die cavity being shaped and dimensioned to squeeze and thin aportion of the sheet in an annular zone in the base region of thedimple; and second die means including dies cooperative to form thedimple into a hollow rivet, the second pair of dies being shaped anddimensioned to position substantially all of said annular thinned zoneoutside of the rivet.
 71. A combination as set forth in claim 70 inwhich the die with said cavity therein is formed with a bore at thebottom of the cavity to permit the crest portion of the dimple to bulgeinto the bore during the operation of forming the dimple.
 72. In anapparatus of the character described for forming a dimple in a sheet ofdeformable material, the combination of: a die with a cavity therein anda punch to force a portion of the sheet into the die cavity, the punchand the wall of the die cavity being shaped and dimensioned to squeezethe metal in an annular zone of the dimple spaced radially away from thecrest of the dimple with consequent extrusion of the sheet materialtowards the crest of the dimple, said punch and die cavity being shapedand dimensioned to provide clearance for the crest of the dimple to bowaway from the punch in response to the squeeze operation.